This invention relates to a method of mixing low density syntactic foam, and more particularly to a method mixing a low density syntactic foam made of glass microspheres held in a rigid matrix with a minimal volume of bonding agent.
For purposes herein, a syntactic foam shall be defined as a material comprised of hollow microspheres adhered together with a bonding agent. A low density syntactic foam is a syntactic foam using only as much bonding agent as is required to bond the microspheres together at their adjacent or contacting edges in a rigid matrix.
Syntactic foams are useful for a number of applications. Their primary advantage is that they afford rigidity and structural strength at very low density compared to conventional structural materials. Syntactic foams may be used as fillers in honeycomb material to add rigidity and strength to the honeycomb structure. Syntactic foams are also formed or molded into the shape of the final part and then covered with a structural skin, such as composite material, to lend strength and desired skin characteristics to the part.
Syntactic foams are also useful as a rigid matrix for holding other desirable materials in a low-density, spatially fixed relationship. For example, syntactic foam made with metallic coated microspheres and formed in a sheet serves as an excellent low density shielding material against electromagnetic radiation. The material can also be mixed with various fibers, flakes and granules for enhanced structural, chemical and/or electrical characteristics. Finally the material affords an excellent model molding compound because of its light weight and stable nature.
Basically, syntactic foam is manufactured simply by mixing hollow glass microspheres with a bonding agent and allowing the bonding agent to cure in the desired shape. However, difficulties immediately arise as one attempts to mix conventional bonding agents with the microspheres. The microspheres are extremely small, on the order of 50 to 150 microns in diameter, and very lightweight so that they take on the characteristic of a fine dust. They are also relatively fragile so that rough handling may cause breakage of the glass microspheres. Accordingly, great difficulty has been encountered in mixing the glass microspheres with the stiff and viscous resins and the other common bonding agents used in the manufacture of syntactic foams.
The conventional solution is to mix a sufficient volume of solvent with a bonding agent to reduce its viscosity to the level at which it can be mixed readily with and completely wet the microspheres in a homogeneous pasty mass or magma without extensive damage to the microspheres in the mixing process. This technique works well to create a homogeneous magma and to minimize breakage of the microspheres, but it creates a host of problems. Perhaps the most serious problem created by excess solvent is that it tends to draw the resin off of the microspheres during solvent evaporation and, because the resin and solvent solution is heavier than the microspheres, it tends to sink to the bottom of the mixing container when the mixer is turned off. To prevent this separation of solvent/resin from the microspheres, the magma must be continually mixed until the solvent moiety is reduced to the point that the mixture becomes stable.
Another problem caused by excess solvent is that a soupy or liquid mixture of resin and solvent in the solid components promotes separation of the solid components into regions of equal density, so that the microspheres tend to float to the top of the mixture, the fibers, flakes or granules float below that, and the solvent and resin solution sinks to the bottom.
The excess solvent in a solvent-rich solution must be evaporated before the magma can be packed into a mold because otherwise the components will separate within the mold and create the same problem as discussed above. This effect makes it difficult to obtain homogeneous batches from batch to batch and also within a particular batch wherein the materials are uniformly and homogeneously distributed throughout the material.
The necessity of evaporating the excess solvent requires more than simply spreading it out over a large surface area and allowing the solvent to evaporate. The material must be continually stirred, otherwise the microspheres float to the top and form a skin which then slows the evaporation of the solvent and creates an undesirable material. The continual stirring is a labor-intensive process because the material must be stirred gently and must be checked continually to ensure that it does not separate and harden before use. The evaporating solvent, of course, is lost, which is wasteful of the solvent and increases the cost of the material, and it requires special provisions to exhaust the evaporating solvent from the work area.
Accordingly, it has long been a need in the art to provide a method of mixing a magma for making syntactic foam with no more than the amount of solvent that is optimum for such a magma, such that the microspheres and the bonding agent can be mixed in a homogeneous magma without damage to the microspheres and without a separate solvent evaporation step.